We have, as defined on Danbooru:
Oral: "Contains oral sex, the stimulation of another's genitals via the tongue and mouth. Cunnilingus refers to pleasing the female sexual organs, while fellatio and irrumatio refer to pleasing the male sexual organs."
Cunnilingus: "Oral stimulation of the female genitals, especially the clitoris. Male counterpart is a fellatio."
Blowjob: "Oral stimulation of the penis, to be distinguished from cunnilingus. Also more formally known as fellatio, and considered a form of oral sex. May be done in conjunction with handjob and ball caress. Note: oral stimulation of the penis may involve the head or the shaft of the penis, and therefore does not automatically imply deepthroat."
Fellatio: "The formal term for a blowjob. It is a sexual act in which someone uses their mouth to stimulate their partner's penis. Not to be confused with irrumatio."
Irrumatio: "A type of oral sexual intercourse performed by actively thrusting one's penis into a partner's mouth and throat."
Deepthroat: "Depictions of fellatio where the whole penis is engorged by the mouth and down the throat."
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So what do we learn from this?
A very messy relationship between the terms.
1. Oral is the master-tag for oral-sex, separated into Cunnilingus for females and... Fellatio and Irrumatio for males. (Why two tags for males?)
2. Cunnilingus is the catch-all-term for vagina-stimulation-orally and only mentions Fellatio as it's counterpart. (Where is Irrumatio?)
3. Blowjob, also known as Fellatio, is the catch-all-term for penis-stimulation-orally; not all blowjobs are deepthroat but all deepthroats are a blowjob. (Merely aliased to Fellatio. Not aliased to either Irrumatio or Blowjob despite supposed catch-all definition)
4. Fellatio, also known as Blowjob, is actually a specific kind of blowjob where the penis is stimulated by another party. (Suppose to be a catch-all term for penis-stimulation according to Blowjob; not really according to itself. Mentioned solely as the counterpart to Cunnilingus, even though Oral defines that position as being shared by Irrumatio)
5. Irrumatio works differently from Fellatio.
6. Deepthroat is a type of fellatio. (The description is wrong because deepthroat can be performed with irrumatio as well)
Well, long-story short, I find it really odd that oral stimulation of the male genitals is separated into 2-4 different tags, while the oral stimulation of the female genitals are in one neat tag.
I really think there should be some sort of catch-all-term/tag for oral stimulation of the penis.